Simple AES encryption decryption with openssl library in C

Question

I want to encrypt a struct containing few String and then decrypt it. I tried following code. The original code is found from the web and it was working perfectly. I change

Answer 1

I would almost go so far as to say this is a problem with OpenSSL. It seems that when the length parameter passed to AES_cbc_encrypt is > AES_BLOCK_SIZE but not an integral multiple thereof (i.e. length mod AES_BLOCK_SIZE != 0), then the last block is encrypted using the initial IV and not the previous block of ciphertext as should be the case for CBC mode.

You can fix this in one of two ways:

Copy your struct to a buffer whose size is an integral multiple of AES_BLOCK_SIZE, or encrypt in two parts - the full blocks, followed by a single partial block. The latter has the advantage of avoiding additional memory usage and can be done as follows:

size_t fullBlocks = inputslength - (inputslength % AES_BLOCK_SIZE);
size_t remainingBlock = inputslength - fullBlocks;

AES_cbc_encrypt((unsigned char *)&ticket, enc_out, fullBlocks, &enc_key, iv_enc, AES_ENCRYPT);
AES_cbc_encrypt((unsigned char *)&ticket + fullBlocks, enc_out + fullBlocks, remainingBlock, &enc_key, iv_enc, AES_ENCRYPT);

You should then be able to decrypt as you currently are without issue. It's worth noting however that you should declare dec_out as the same size as enc_out, because you're currently overrunning the dec_out buffer when decrypting.

Edit:

I raised this as a bug in OpenSSL: https://rt.openssl.org/Ticket/Display.html?id=3182&user=guest&pass=guest and whilst there is some argument over whether this is actually a bug, or just (undocumented) undefined behavior, the general consensus is that the EVP routines should be used instead, rather than these low-level functions.